A reflection-absorption infrared spectroscopy study of the adsorption of dioxygen species on a silver surface

Abstract

Two, adsorbed molecular-oxygen species are identified on a polycrystalline silver surface using reflection-adsorption infrared spectroscopy (RAIRS). One oxygen species is a peroxo (O 2− 2) species which is identified from its vibrational frequency (983 cm −1) and which stands up on the surface. Its heat of adsorption, measured from adsorption isosteres, varies with oxygen coverage and is given by ΔH (ads)=14.5–18.7 θ kcal/ mol where θ is the coverage and θ=1 is measured from the integrated intensity of the 983 cm −1 RAIRS peak for a sample temperature of 90 K in the presence of 145 Torr of oxygen. The second oxygen species is oriented with its OO bond parallel to the surface, has an OO stretching frequency of 622 cm −1 and is postulated to be the precursor to dissociative oxygen adsorption. Its heat of adsorption is also coverage dependent being given by ΔH (ads)=14.0–16.3 θ kcal/ mol where here θ=1 is taken as the maximum integrated peak intensity for an oxygen pressure of 250 Torr at a sample temperature of 90 K. The variation in coverage for both species as a function of oxygen pressure is well described by a Temkin isotherm in accord with their coverage-dependent heats of adsorption.